1. Field of the Invention
The present invention relates to a method and a device for measuring the true rate of penetration of an equipment progressing in a well. The invention can be applied in the petroleum field for determining the true rate of penetration of any well equipment, be it a drill bit or a measuring sonde for example. Knowing the true rate of penetration is of great importance for drillers, but also for geologists who need to know with precision the strata of the subsoil crossed through by a well.
2. Description of the Prior Art
Examples of the prior art in the field of determination of the measurement of the rate of penetration of a tool in a well are described in U.S. Pat. Nos. 2,688,871, 3,746,102 and 4,567,759, in European patent application EP-289,068 and in French patents 2,038,700 and 2,593,606, etc.
The rate of penetration of a tool in a well is often obtained indirectly through measurements performed at the surface. In the case of a drill bit, a standard method consists in measuring at the surface the rate of penetration into the ground of the string which supports the drill bit, as well as another parameter such as stress in the string or else, at the well bottom, the weight applied to the drill bit.
The results obtained with these type of methods are generally not very accurate. In fact, the string consists of more or less flexible drillpipes or tubes. It is therefore relatively elastic and the tension varies as a function of the length thereof. Consequently, corrections have to be introduced so as to take into account the length variations thereof, which are all the more considerable since the well is deep. The large friction between the string and the walls, as well as the influence of the different thrust loads exerted on the the equipment as a whole penetrating into the well should also be taken into account. Determining the rate of penetration therefore becomes very complex if all the factors which are likely to alter the values of the parameters introduced in the calculation are taken into account, insofar as they are known with precision.
Another method used with drilling operations consists of including in the equipment at the bottom of the hole several accelerometers for measuring the components of the acceleration of the equipment. The accelerations are integrated to obtain the rate of penetration according to the direction of the well. This method actually allows a direct measurement of the rate of penetration to be obtained. However, the implementing thereof is made difficult by the very large vibrations of the bit at the bottom of the hole, and also by the fact that the the signals provided by the accelerometers are subject to drift which if not compensated distorts the results.